Assessment of Different Strategies for Composting of the Two-Phase Olive Mill Solid Waste: A Demonstrative Scale

Thousands of tons of two-phase olive mill solid waste (2P-OMSW) are generated annually, necessitating effective valorization strategies. Composting has been widely explored as a management approach; however, the extended processing time required for these residues poses a significant challenge for the olive industry. In this study, a forced aeration system combined with a semipermeable cover was implemented at a demonstrative scale to enhance the composting process and reduce its duration. Additionally, process optimization was evaluated through a two-stage composting strategy. In stage I, compost preconditioning was carried out using two types of manure (poultry and cow). In stage II, a bioaugmentation process was introduced using the edible fungus Pleurotus eryngii. The composting of 2P-OMSW under forced aeration and a semipermeable cover lasted 90 days. During the composting process, physicochemical parameters, total phenol content, microbial analysis, and phytotoxicity bioassays were measured to evaluate the efficiency and quality of the final compost. In stage I, poultry manure proved to be more effective than cow manure, resulting in a lower C/N ratio (<25%), higher nitrogen, phosphorus, and potassium content, and a greater reduction in total phenol content (>70%). In stage II, bioaugmentation significantly enhanced the removal of heavy metals, particularly zinc (Zn) and copper (Cu). Both final composts, obtained within 90 days, exhibited enriched nutrient content, stabilized nonphytotoxic organic matter, and low heavy metal concentrations. The findings highlight the potential of a forced aeration system combined with a semipermeable cover as an effective strategy for composting 2P-OMSW. This approach facilitates the transformation of 2P-OMSW into high-quality compost, making it suitable for use as an organic amendment or fertilizer in agricultural systems. Furthermore, it allows for the management of this residue within a relatively short time frame.